1. Field of the Invention
The present invention relates to the work-up of a reaction mixture formed by ammoximation of a ketone by means of hydrogen peroxide and ammonia, wherein the work-up process comprises at least one membrane separation step.
2. Background of the Invention
Numerous patent applications and patents describe the ammoximation of ketones, in particular alkanones and/or cycloalkanones, by means of hydrogen peroxide and ammonia over a heterogeneous catalyst system which comprises at least one component composed of the elements titanium, silicon and oxygen.
Examples which may be mentioned here are EP-A-0 299 430, EP-A-0 564 040 and U.S. Pat. No. 5,637,715.
In general, the catalyst used is a microporous or mesoporous titanium zeolite, with the titanium silicalite TS1 being particularly suitable for ammoximation. Furthermore, in the case of bulky ketones such as alkanones or cycloalkanones, it is advantageous to supplement the catalyst system with further components. Thus, DE 195 21 011 describes and claims amorphous silicates, DE 100 47 435 describes and claims acidic solids and DE 101 03 581 describes and claims ammonium ions as cocatalyst.
As described in DE 100 47 435 and DE 101 03 581, the reaction of bulky (cyclo)alkanones such as cyclododecanone proceeds particularly quickly and selectively in polar organic solvents which are completely or partially miscible with water, in particular in short-chain alcohols having from 1 to 6 carbon atoms.
The ammoximation occurs in two substeps comprising hydroxylamine formation (1) and oximation (2). Water is firstly introduced by means of an aqueous hydrogen peroxide solution and, secondly, water is formed in stoichiometric amounts as reaction product in the two substeps.
In addition, water is also formed in the unproductive decomposition of hydrogen peroxide and hydroxylamine, formally shown in the secondary reactions (3) and (4) of the following reaction scheme formulated for cyclododecanone (CDON) as an example:
NH3+H2O2xe2x86x92H2O+NH2OHxe2x80x83xe2x80x83(1)
NH2OH+CDONxe2x86x92CDON oxime+H2Oxe2x80x83xe2x80x83(2)
2NH2OH+H2O2xe2x86x924H2O+N2xe2x80x83xe2x80x83(3)
2H2O2xe2x86x922H2O+O2xe2x80x83xe2x80x83(4)
Consequently, the water content of the reaction mixture increases during the reaction. If large alkanones or cycloalkanones such as cyclododecanone are to be ammoximated, the solubility of the corresponding oxime in the reaction mixture drops sharply with increasing water content.
A particular objective in the reaction of large cycloalkanones is therefore to restrict the amount of water during the reaction as much as possible. According to DE 100 47 435 and DE 101 03 581, this is achieved, for example, by ammonia being used as dry gas and hydrogen peroxide being used as a very concentrated solution (usually  greater than 30% by weight). It is also advantageous for the alcohols used as solvent to contain, at the beginning of the reaction, no more water than is present in the azeotrope after distillation.
If the alcohol is to be used a number of times in the process, the amount of water introduced during the reaction has to be separated off again in the work-up.
In most patent applications, the synthesis of the catalyst system, its activation and the ammoximation reaction itself are the focal points of the investigations. On the subject of the work-up, the abovementioned documents state in general terms that the usually pulverulent catalyst, in general a titanium silicalite, is separated off via a filter or a pressure filter. Conversion and selectivities are subsequently determined by GC analysis and the peroxide consumption is determined directly on the reaction solution by redox titration. If the reaction mixture is worked up further, purification by distillation and/or extraction are chosen for this purpose.
In the European patent applications EP-A-0 690 045 and EP-A-0 735 017, ARCO Chemical Technology describes a multistage process for the synthesis of caprolactam in which the ammoximation of cyclohexanone is carried out using hydrogen peroxide from the reaction of isopropanol and oxygen. For the ammoximation of cyclohexanone, any suitable work-up process is claimed in general terms. EP-A-0 735 017 mentions distillation and extraction as possibilities without these two methods being placed on a concrete basis by means of experimental data or examples.
Complete separation of solvent, starting material and product by distillation after the ammoximation stage, as envisaged in U.S. Pat. No. 5,451,701 and EP-A-0 690 045, might well be possible in the case of cyclohexanone oxime. After the solvent and water have been distilled off, cyclohexanone (b.p. 155xc2x0 C./1013 mbar) and cyclohexanone oxime (b.p. 206-210xc2x0 C./1013 mbar) can be separated from one another by distillation. This distillation is advantageously carried out under reduced pressure.
However, a method involving purely distillation is no longer suitable for the ammoximation of macrocyclic ketones such as cyclododecanone. The separation of ketone and oxime by distillation becomes increasingly difficult as the ring size increases, and, in addition, the high distillation temperatures even in a high vacuum result in a considerable degree of decomposition. Cyclododecanone oxime, for example, can no longer be distilled without decomposition.
A number of publications mention extraction for the work-up. In EP-A-0 208 311, example 1, Montedipe describes the reaction of cyclohexanone and work-up of the ammoximation product of cyclohexanone without alcohol as solvent in a three-phase mixture (organic-aqueous-solid) comprising cyclohexanone as organic phase, 32% strength by weight aqueous ammonia and 32% strength by weight aqueous hydrogen peroxide as aqueous phase and pulverulent titanium silicalite as solid catalyst. For the work-up and removal of the catalyst, the organic phase is taken up in toluene, the aqueous phase is extracted a number of times with toluene and the catalyst is separated off by filtration.
In the patent U.S. Pat. No. 4,794,198 and the European patent application EP-A-0 267 362, an organic solvent, for example an ether, is added to the cooled reaction mixture after the ammoximation and the cyclohexanone and the corresponding oxime are extracted by means of this.
According to EP-A-0 496 385, Enichem firstly distills off an ammonia-containing azeotrope of solvent, tert-butanol and water. The oxime and alkanone are subsequently washed out of the distillation bottoms by means of toluene in an extractor.
The above-mentioned work-up processes have, in particular, two disadvantages:
The ketone used in each case and its corresponding oxime firstly become increasingly similar in terms of their extraction behavior as the size of the molecule increases and although they can be removed from the reaction mixture together with the extractant used, they can be separated from one another only incompletely, if at all. A ketone-free oxime can be obtained in this way only in the case of complete conversion of the ketone.
However, it is known from numerous documents, for example DE 100 47 435 and DE 101 03 581, that the reactivity of ketones in the ammoximation reaction decreases with increasing size. Complete conversions of bulky ketones are only possible at long reaction times and with a high peroxide consumption (=poor peroxide selectivity).
A further disadvantage of the abovementioned work-up methods is that the distillation of the solvent mixture requires a large amount of energy.
Since low-boiling, short-chain alcohols having preferably from 1 to 6 carbon atoms are preferably used in ammoximation processes for large alkanones and cycloalkanones, the removal of the water of reaction by rectification or distillation results in the total amount of alcoholic solvent going over at the top of one or more columns. This means that the enthalpy of vaporization for the total amount of solvent has to be introduced. In the subsequent condensation of the solvent, this energy has to be passed to a cooling medium. Despite the use of heat exchangers, these processes have a considerable energy consumption which has a very adverse effect on the economics of these processes.
In addition, the above-described solvent distillation is unsuitable in the case of incomplete ketone conversions and/or in the case of incomplete separation of the oxime from the reaction mixture, since these compounds accumulate in the stripping section or the bottom of the column and can crystallize out there.
It is therefore an object of the invention to find an energy-saving work-up process for the ammoximation of ketones, in particular bulky alkanones and cycloalkanones having, in particular, from 8 to 20 carbon atoms, in which
the catalyst is separated off after the reaction,
the oxime is isolated selectively from the reaction mixture,
the water of reaction is separated off from the solvent in an energy-saving manner and
the solvent which remains is returned to the process, and which can be carried out even in the case of incomplete conversion in the ammoximation.
It has surprisingly been found that the solubility of ketone oximes, in particular many alkanone oximes and cycloalkanone oximes in short-chain alcohols is highly temperature dependent. The oximes can thus be separated very selectively from the reaction mixture even at incomplete ketone conversion by means of suitable temperature conditions. Even a single-stage crystallization achieves oxime purities of above 99% by weight, which are sufficient for direct further use of the oximes without additional purification steps, for example in a Beckmann rearrangement.
In addition, it has surprisingly been found that water can be removed very selectively and in an energy-saving manner from the remaining mother liquor by preparation or vapor permeation through membranes and that this work-up step is also possible in the case of incomplete ketone conversion and the resulting presence of ketones.
The invention accordingly provides a process for the work-up of a reaction mixture formed by ammoximation of a ketone, in particular an alkanone or cycloalkanone having, in particular, from 8 to 20 carbon atoms, by means of hydrogen peroxide and ammonia in homogeneous solution over a titanium-containing catalyst, which comprises the substeps
removal of the catalyst,
selective separation of the oxime formed in the ammoximation from the reaction mixture,
removal of water of reaction and
recirculation of the solvent,
wherein the work-up process is carried out, even in the case of incomplete ketone conversion, by use of at least one membrane separation step.
Accordingly, the present invention is directed to a process for the work-up of a reaction mixture containing an oxime formed by ammoximation of a ketone with hydrogen peroxide and ammonia in a homogeneous solution containing a solvent over a titanium-containing catalyst, comprising the steps of:
removing the catalyst,
separating the oxime from the reaction mixture,
removing water of reaction from the reaction mixture, and
recirculating the solvent,
wherein at least one of said steps comprises a membrane separation.